In armor-piercing ammunition, a very high velocity and stability of the projectile in flight are essential for the round to achieve its objective in terms of range, accuracy and target destruction. Stability of the projectile may be accomplished by different means, such as by adding fins, or, in the case of this invention, spinning the projectile above a predetermined angular velocity. Weight distribution, geometrical shape, and flight conditions contribute to the desired stability.
Conventional design includes a sabot carrying a subprojectile which is spun to the required angular velocity by means of a rotating band on the sabot engaging the rifled gun tube. This engagement causes the sabot and thus the subprojectile to rotate as they pass through the barrel of the gun. Difficulties have been encountered when a tracer is added to the subprojectile. The simplest method of igniting the tracer is by means of the hot propellant gases which launch the projectile. This requires a hole in the base of the sabot to expose the tracer material to the hot gases.
Using hot propellant gases to ignite the tracer unfortunately allows gases to penetrate between the subprojectile and the sabot. This has various adverse effects such as alteration of friction conditions, formation of a gas bearing between the two elements, blow-by of gases inside the round, or reduction of setback forces causing the subprojectile to be prematurely separated from the sabot. All of these can cause the subprojectile to fail to reach its critical spin rate, causing it to corkscrew or yaw. These undesirable flight characteristics result in not all ammunition firing at the same velocity or with the same degree of accuracy.